Cellophane is so thin and flimsy. What makes the wrapper on my Ritz crackers crackle and crinkle like a firework?
In: Physics
Fun fact: Candy wrappers are rarely cellophane nowadays, but are specifically engineered to make the same loud sounds because that’s just what people got used to.
Is cellophane what makes diapers so loud?
It is made of a more rigid material than other bags, think of it being closer to aluminum foil or a large baking sheet that is a bit wobbly or warped, once it has a deformity or crease it will continue up to a point of stability making sharp noises. That stiffness helps transfer energy around large areas to specific points of tension and weakness.
The bag wants to stay in its original shape. When you change its shape, it reacts by releasing energy, letting out clicks, heat and vibrations. This is the bag crying.
The more you change its shape, the more it cries. A bag is not one creature, but many creatures bonded together, and each shape change is breaking some of the bonds between them.
If a bag is smooth at the start, it will cry a little when you crumple it. If it has been crumpled many times and it has many creases it will usually cry more than it did when it was first crumpled.
Bags with different thicknesses of material will cry differently. Thick bags (like the bag of the crisp or of the cracker), their individual cries can be louder, but thin bags with many creases have many small cries which, when multiplied, can also be deafening, especially when hungover
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Sound is the vibration of air, cellophane and other thin film like materials function like membranes in that their large surface area pushes a lot of air around when they move and deform, the elasticity of the materials and some of their other features such as the creases and folds make for some rather ‘violent’ motions within the material itself that make it momentarily push even more air around when you squish it around.
The membrane in your loud speaker is also thin and flimsy, move it around fast enough and it can push enough air to get pretty loud.
Packaged food manufacturers have certain requirements for their food packaging. Some of these are best solved by cellophane – it retains shape well (so it looks good on shelf), it’s largely water and gas impermeable (so it keeps things fresh), and is shiny (again, looks good on shelf). Strangely, the sound plays a role too – for many products, consumer testing shows that people expect a certain level of crinkling but more or less is not good – if it doesn’t crinkle enough, it is interpreted as “old, stale, old-fashioned, earthy”, if it crinkles more than expected “it’s annoying, artificial, distracting”…..
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